Public attention is focused on polymer alloys having the characteristics which are not obtained by a single polymer but obtained by blending two or more polymers which are incompatible with each other in a usual state. Especially, in the case where two or more polymers form a ultra-micro-phase separation structure, a polymer alloy on which the characteristics of each resin are reflected can be obtained. For example, an amorphous polymer having high heat resistance is added to an amorphous polymer which has good moldability but low heat resistance to form a polymer alloy, whereby a polymer alloy having good moldability and high heat resistance can be manufactured. In addition, unlike copolymers such as block copolymers or random copolymers, troublesome copolymerizing operations are not required in the production of the polymer alloy.
Conventionally, a kneading method is used as a method for producing a polymer alloy having a ultra-micro-phase separation structure by blending two or more polymers which are incompatible with each other in a usual state. In order to obtain a satisfactory ultra-micro-phase separation structure, it has been regarded as essential to use some compatibilizing agent. As the compatibilizing agent, one corresponding to a raw polymer must be selected. However, this selection is not easy, it is difficult to obtain a polymer alloy forming a ultra-micro-phase separation structure and having desired characteristics and, also, there are combinations of polymers for which good compatibilizing agent has not been found so far.
Meanwhile, in Japanese Kokai Publication Hei-2-134214, a method is disclosed in which two types of polymers are melted using supercritical gas which is present in a gas state at ambient temperature and pressure or a mixture of supercritical gas, these components are thoroughly mixed for a plenty of time until the viscosity of the polymer mixture is decreased by at least 10%, then the melt mixture is cooled sufficiently, taking much time to continue mixing until the viscosity of the melt mixture of the polymers reaches again at least the original value and then the pressure in the mixing container is rapidly released to produce a polymer alloy micro-dispersion phase separation structure. Also, in Japanese Kokai Publication Hei-10-330493, a method is disclosed in which a solvent which is a liquid state at ambient temperature and pressure is changed to a high temperature and high pressure fluid to make incompatible two or more polymers compatible with each other and then the pressure in the system is rapidly dropped to vaporize the solvent, thereby producing a polymer alloy having a ultra-micro-phase separation structure 100 nm or less in size.
However, these methods of producing a polymer alloy involve a cooling process using the so-called adiabatic expansion in which process the pressure of supercritical gas or a mixture containing supercritical gas is suddenly released or is dropped suddenly from the pressure condition to thereby vaporize the high pressure and high temperature fluid during the course of the process; therefore, a large number of air bubbles are generated in the resulting polymer alloy. A troublesome defoaming process is required to obtain a transparent molded article by using such a polymer alloy having air bubbles and, also, there is the case where the ultra-micro-phase separation structure of the polymer alloy is broken by the defoaming process, which considerably limits the range of the applications of these methods. There is also the problem that it is hard to scale up the process of vaporizing a solvent suddenly; therefore, the industrialization of the process is difficult.
A method for producing a polymer alloy is disclosed in Japanese Kokai Publication Hei-6-234861, the method using at least one block copolymer or graft copolymer in a supercritical fluid. However, in this method, a pressured polymer alloy is expanded by passing it through a fine nozzle; therefore, foaming is easily caused, giving rise to the problem that a process of removing air bubbles is inevitable. Also in this method, after glass beads to which a raw resin is stuck are packed in a column, the supercritical fluid is flown through the column to mix the resin with dissolving the resin; therefore, the ratio of a composition is determined by the solubility ratio of each resin. Further, because the amount of a resin which can be treated is small and a raw resin cannot be supplied continuously, resulting in small throughput.